The concept of total ankle arthroplasty has a long and relatively unsuccessful history. Only recently has total ankle arthroplasty regained some recognition as a viable treatment for limited indications. Replacement of the ankle joint is particularly problematic due to the relatively small articular surfaces, complex biomechanics, and limited access to the joint during replacement, and wide variation in patient candidacy. These factors have led to post-operative complications such as loosening, subsidence, pain, and prosthetic wear. In addition to these technical difficulties, regulatory agencies have classified ankle prosthetics in a manner substantially limiting scientific progress in ankle replacement due to the financial burden of obtaining market clearance for such devices. What is needed is an ankle prosthetic system or kit that can sufficiently address all types of surgical candidates considered for total ankle replacement. The kit must provide means to adjust the fixation and articular adjustment such that modifications to the “fit” and “function” of the prosthesis can be made intraoperatively or interoperatively (i.e. within the same or in a separate operation). Further the kit must provide means for legal distribution of the device depending on the legal and/or regulatory constraints place on such devices and the geographic location of use.
Other ankle prosthetics currently marketed include the following:
CompanyDevice NamePatent(s)TypeDePuyAlvine/Agility5,326,365SemiEndoTechBP (Buechel, Pappas)4,309,778UnconstrainedLINKS.T.A.R.unknownUnconstrainedTornierUnknown5,824,106:Unconstrained6,183,519
The key element in the chart above is the type of prosthesis. Two types of prosthetics are generally available: Semi-constrained, and unconstrained. Both types of prosthetics make use of a 3-component design: upper, middle, and lower component (tibial, bearing, and talar component, respectively).
A semiconstrained prosthesis such as the Alvine device provides for a tibial fixation component (metal), which provides firm attachment to the distal end of the tibial bone. A talar component provides firm attachment to the proximal end of the talar bone, and provides on its upper or proximal side a generally convex surface for articulation. Into the tibial component fits a UHMWPE bearing that intimately fits into a socket formed to receive the bearing. The two components fit together such that no motion is considered between the bearing and the tibial component. The underside of the bearing provides a generally concave surface to articulate with the convex surface of the talar component. The radii of curvature of these curved surfaces are mismatched such that all motions present in a nature ankle can be at least partially replicated. These motions include plantar/dorsiflexion, rotation about the tibial axis, medial/lateral translation, and anterior/posterior translation. Rotations in the frontal region are not well supported as there is little curvature in this region. These motions (they can occur actively) lead to edge loading, causing higher stress and greater propensity for wear. Also, as the articular surfaces are designed for mismatch, even under optimum implant positioning and loading, higher stress will be seen at the contact point due to the point loading associated with mismatched radii.
The unconstrained prosthetics are all generally the same in function. They are similar to the semiconstrained prostheses except that there is designed into the prosthesis the potential for motion between the tibial tray component and the bearing. There is no intimate fit between the bearing and the tibial component; the tibial component has a flat undersurface and the bearing has a simple flat upper surface so that translation and rotation are allowed at this interface. Further, the curvature of the interface between the talar component and the bearing are matched, so there is a large contact surface area and optimized contact stress (reduced wear). This match articulation can be accomplished because other motions are allowed for between the tibial and bearing components. It has been clearly shown with clinical history in all joints that if these motions are not allowed for, the force must be absorbed at the implant bone interface, and can lead to a greater propensity for loosening.
Another known device is described in U.S. Pat. No. 5,824,106 (hereafter the '106 Patent) to Fournol, which describes an ankle prosthesis having a tibial component with an articular surface that slides on an intermediate element, which in turn has a recess for accommodating a protrusion or lug of a talar component. The '106 Patent, aside from differing substantially from the embodiments of the present subject matter, does not contemplate selecting the intermediate component from a plurality of components, each having a differently shaped recess for meeting a recipient's ambulatory needs and for overcoming the legal obstacles of different regions.
It has been commonly considered to have a kit that allows a surgeon to select from varying sized or thickness of bearings. However, it has not been known thus far to have the option of selecting from a plurality of bearings that allow one to control, in varying amounts, the amount of motion allowable between the potential articular surfaces.
Further, it must be noted that the Food and Drug Administration (FDA) currently classifies unconstrained ankle prostheses as class III devices. Under FDA regulations, class III devices require Pre-Market Approval (PMA) prior to distribution in the US. This further means the clinical data must be provided that can substantial the efficacy of the new medical device. This clinical data is increasingly expensive to develop and in most cases prevents a particular project from being financially viable. The semiconstrained device is a class II device, and typically does not require PMA, but substantial equivalency to a currently approved device. There are currently no legally marketed ankle prostheses in the US.